4606
J. Phys. Chem. 1996, 100, 4606-4611
Electron Spin Resonance and Electron Spin Echo Modulation Studies of Catalytic Ethylene Dimerization on Palladium-Exchanged Silicoaluminophosphate Type 5, 8, and 11 Molecular Sieves Martin Hartmann* and Larry Kevan* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: October 13, 1995; In Final Form: December 20, 1995X
Oxygen-pretreated, palladium-exchanged silicoaluminophosphate SAPO-11, SAPO-5, and SAPO-8 are catalytically active for ethylene dimerization. The catalytic activity of the silicoaluminophosphates is shown to be due to Pd(I) species and is greatly dependent on the large channel size of the supporting materials which vary from 10-ring (SAPO-11) to 12-ring (SAPO-5) to 14-ring (SAPO-8) sizes. The selectivity for the formation of n-butenes is influenced by the reaction temperature and the channel size, showing that lower temperatures and larger channel diameters favor n-butene formation. Electron spin resonance studies show isolated palladium(I) species A (g| ) 2.963 and g⊥ ) 2.141) and B (g| ) 2.678 and g⊥ ) 2.078) after activation in SAPO-5 and SAPO-11. In SAPO-8 only species A′ (g| ) 2.91 and g⊥ ) 2.12) can be observed. After ethylene adsorption in all materials, species G (g| ) 2.245 and g⊥ ) 2.003) can be seen, which transforms to species H (g| ) 2.421 and g⊥ ) 2.033) and K (g| ) 2.641 and g⊥ ) 2.076) during the reaction with simultaneous butene formation detected by gas chromatography. Electron spin echo modulation analysis allows the assignment of species G to a Pd(I)-C2D2 complex and species H to a Pd(I)-C4D8 complex. This work shows that in silicoaluminophosphates monovalent palladium cations coordinate ethylene and are catalytically active for ethylene dimerization.
Introduction
Experimental Section
In the past decade the synthesis and characterization of a new group of aluminophosphate (AlPO4-n) and silicoaluminophosphate (SAPO-n) molecular sieves have been achieved.1 By incorporation of silicon atoms into the aluminophosphate framework SAPO materials have ion-exchange capacities, and catalytically active transition metal ions can be incorporated. Pd-loaded catalysts are widely used for various reactions such as ethylene dimerization2 and CO hydrogenation.3 Pd(II) ions can be incorporated into zeolites and silicoaluminophosphates by liquid phase and solid state ion exchange. Depending upon the pretreatment, Pd(I) and Pd(III) oxidation states can occur in zeolites.4-9 However, in silicoaluminophosphates (SAPO’s) Pd(I) seems to be the only paramagnetic oxidation state stabilized, owing to the lower negative framework change. Previous electron spin resonance (ESR) and electron spin echo modulation (ESEM) studies have characterized Pd(I) locations and adsorbate interactions in SAPO-11 and SAPO-5 molecular sieves.10-13 These molecular sieves have a large straight channel which is formed by 10 T atoms (Al, P, Si) and 12 T atoms. SAPO-8 which, can be obtained from SiVPI-5 by thermal treatment,14,15 has 14-T atom channels with a 0.87 by 0.79 nm eliptical shape. Despite the ongoing discussion about whether Si can be incorporated into the structure,16-19 we found a significant ion-exchange capacity which suggests successful incorporation.20 In this study Pd(II) was incorporated into H-SAPO-11, H-SAPO-5, and H-SAPO-8 via solid state ion exchange, and after an activation treatment ethylene dimerization was carried out in a static reactor. ESR and ESEM measurements were used to determine the interaction of palladium ions with reactant and product molecules.
Synthesis and Ion Exchange. SAPO-5 and SAPO-11 molecular sieves were synthesized according to a Union Carbide patent21 with some modifications made in our laboratory.22,23 The as-synthesized SAPO-5 and SAPO-11 materials were heated in flowing pure oxygen for 24 h at 873 and 823 K, respectively. By this procedure the organic templating agent is removed and H-SAPO-n (n ) 5, 11) is formed, in which the framework negative charge is balanced by H+. SAPO-8 was obtained from SiVPI-5 by heating the powder to 130 °C for 12 h in air. SiVPI-5 was synthesized using 1,1′,1′′-nitrilotri-2-propanol. (TIPOA) as a template. The details of the synthesis are described elsewhere.20 Solid state ion exchange was performed by mixing 1 g of the SAPO material with 0.02 g of Pd(NH3)4Cl2‚H2O (Alfa) and subsequent heating of the homogeneous looking powder in oxygen to 673 K for 18 h. The reaction product was slowly cooled to room temperature and ground to a fine powder. After this treatment the sample is brown and denoted PdH-SAPOn. Sample Treatment. For ESR and ESEM measurements PdH-SAPO-n was loaded into 3 mm o.d. by 2 mm i.d. Suprasil quartz tubes and evacuated to a residual pressure of 1 × 10-4 hPa overnight at 773 K. The samples were then heated at 773 K in 400 hPa of oxygen for 6 h and subsequently evacuated for 18 h at the same temperature. These samples are called “activated” samples. The activated samples were then exposed to ethylene-d4 (Cambridge), 1-butene, and cis/trans-2-butene (Union Carbide) at 100 hPa (75 Torr). The adsorbates were purified by repeated freeze-pump-thaw cycles before use. Gas Chromatography. A sample of 100 mg was placed on a sintered glass disk inside a glass reactor of total internal volume of 51 cm3. The silicoaluminophosphates were activated as described above, and then 400 hPa (300 Torr) of ethylene was filled into the reactor. The gas phase over the SAPO
X
Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4606$12.00/0
© 1996 American Chemical Society
Ethylene Dimerization on Pd-Exchanged SAPO’s
J. Phys. Chem., Vol. 100, No. 11, 1996 4607
Figure 1. ESR spectra at 77 K of (a) PdH-SAPO-11, (b) PdH-SAPO5, and (c) PdH-SAPO-8 “activated” at 773 K.
materials was analyzed periodically by withdrawing an aliquot into a Varian Model 3300 gas chromatograph equipped with a thermal conductivity detector. A 6 ft column of 0.085 in i.d. packed with 0.19 wt % picric acid supported on a 80/100 mesh graphic GC support was used. All runs were conducted isothermally at 313 K. Spectroscopic Measurements. All ESR spectra were recorded with a Bruker ESP 300 X-band spectrometer at 77 K. The magnetic field was calibrated with a Varian E-500 gauss meter. The microwave frequency was measured by a HewlettPackard HP 5342A frequency counter. ESEM spectra were measured at 20 K with a Bruker ESP 380 pulsed ESR spectrometer. Three pulse echoes were measured by using a (π/2-τ-π/2-T-π/2) pulse sequence as a function of time T to obtain the time domain spectrum.24 The deuterium modulation was analyzed by a spherical approximation for powder samples in terms of N nuclei at distance R with an isotropic hyperfine coupling Aiso.25 The best fit simulation of an ESEM signal is found by varying the parameters until the sum of the squared residuals is minimized. Results Electron Spin Resonance Studies. After solid state ion exchange the samples are brown and ESR silent. However, an activated sample, which is light gray, produces an ESR signal. Figure 1 shows the ESR spectra at 77 K of (a) PdH-SAPO-5 and (b) PdH-SAPO-11 after activation, which includes dehydration, oxidation, and subsequent evacuation at 773 K. The spectra in both materials show species denoted A (g| ) 2.963 and g⊥ ) 2.141) and B (g| ) 2.678 and g⊥ ) 2.078). Similar ESR spectra have been reported for various palladiumexchanged zeolites.8,9,26 Previously, these two signals were assigned to Pd(I) obtained by thermal reduction and ascribed two different environments in the framework.10,12 By phosphorus31 nuclear modulation it was found that Pd(I) species A is located in site I in a hexagonal prism site and Pd(I) species B is located at site II* near a 6-ring window bordering the large channels of the molecular sieve structure.27 In activated PdHSAPO-8 only one species denoted A′ (g⊥ ) 2.12 and g| ) 2.92) is detected (Figure 1c) and ascribed to isolated Pd(I). Adsorption of n-Butenes. The adsorption of cis-2-butene into PdH-SAPO-5 at room temperature (Figure 2b) leads predomi-
Figure 2. ESR spectra of PdH-SAPO-5 after exposure to cis-2-butene.
nantly to the formation of species C (g| ) 2.442 and g⊥ ) 2.032) and to a smaller concentration of species D (g| ) 2.30) and E (g| ) 2.370) with a common g⊥ ) 2.08. The formation of another species with g ) 1.96 is also detected, which is ascribed to a framework defect in the SAPO-5 structure.22 Keeping the sample at room temperature for 24 h (Figure 2c) does not change the spectrum, but further heating of this sample to 353 K (the temperature used in the gas chromatograph) for 4 h causes species F (g| ) 2.678 and g⊥ ) 2.080) to appear, which has essentially the same g values as species B. After heating the sample to 353 K for 20 h, the intensity of species C decreases with a simultaneous increase of the intensity of species F. The same signals are also found in PdH-SAPO-11 and PdHSAPO-8 and after adsorption of the other butene isomers. Note that the intensity of the framework defect center at g ) 1.96 also increases with increasing temperature. Ethylene Dimerization. The ESR spectra of activated PdHSAPO-11 measured after ethylene adsorption are shown in Figure 3. After 4 h at room temperature (Figure 3b) several species, denoted as G (g| ) 2.003 and g⊥ ) 2.245), H (g⊥ ) 2.032 and g| ) 2.421), and K (g⊥ ) 2.076 and g| ) 2.652), are observed. The development of species G is noticeable right after ethylene adsorption. Pd(I) species A is still visible after ethylene adsorption. The ESR spectra develop slowly with time even at room temperature; species H and K increase while species G decreases. After 20 h at 353 K, species C also decreases with a further increase of species H, which was also observed after n-butene adsorption. The same species are also observed in PdH-SAPO-5 and PdH-SAPO-8. A summary of the different Pd(I) species found in PdH-SAPO-n (n ) 5, 8, 11) in connection with ethylene dimerization is found in Table 1. Electron Spin Echo Modulation Studies. The ESEM analysis of the spectra of PdH-SAPO-5, -8, and -11 with adsorbed C2D4 recorded at the magnetic field set to g⊥ ) 2.01 confirm that species G is due to Pd(I) ions interacting with four equivalent deuteriums at a distance of 0.35 nm and indicate a typical π-bonded configuration. At higher temperature, when
4608 J. Phys. Chem., Vol. 100, No. 11, 1996
Hartmann and Kevan
Figure 3. ESR spectra at 77 K of PdH-SAPO-11 after exposure to ethylene in a static reactor.
TABLE 1: Palladium(I) Species in SAPO Materials Detected during Ethylene Dimerization PdH-SAPO-11 PdH-SAPO-5 PdH-SAPO-8 treatment activation
code
A B A′ adsorption of C n-butenes D E F adsorption of G ethylene H K
g|
g⊥
g|
g⊥
2.963 2.678
2.141 2.078
2.963 2.678
2.141 2.080
2.421 2.300 2.370 2.641 2.245 2.421 2.641
2.032 2.080 2.080 2.076 2.003 2.032 2.076
2.442 2.251 2.369 2.678 2.257 2.421 2.652
2.032 2.081 2.082 2.080 2.007 2.034 2.076
g|
g⊥
2.92 2.497
2.12 2.039
2.91 2.251 2.481
2.12 2.004 2.041
species G, H, and K are observed, strong echo signals and deuterium modulation are obtained at a magnetic field set at g⊥ ) 2.03. If signal G also contributes to this modulation, the ESEM analysis would be ambiguous. But field sweep experiments show that at g ) 2.421 an echo signal can be observed, but at g ) 2.65 (signal K) the echo has disappeared. To minimize overlap with other signals, the modulation was measured at g| (G) ) 2.421. As shown in Figure 4, the simulated spectra are consistent with our catalytic and ESR results that species H may be attributed to Pd(I) complexed with one butene molecule with eight deuteriums at a minimal Pd-D distance of 0.34 nm. Gas Chromatography Studies. Figure 5 shows the ethylene dimerization activity of PdH-SAPO-5, PdH-SAPO-11, and PdH-SAPO-8 in comparison to the parent materials H-SAPO-n after activation of the same samples. While the parent materials show almost no dimerization activity, the activity in the presence of Pd(I) is strongly enhanced, confirming that Pd(I) participates in the formation of the active sites. Because of the low Pd concentration and the low turnover, the different materials were compared after a reaction time of 24 h. Analysis of the gas phase indicates that ethylene is dimerized to isomers of n-butene with the additional formation of side products such as isobutene, propane, propylene, and butane. At 100 °C in PdH-SAPO-5 and PdH-SAPO-11 the selectivity for the
Figure 4. Experimental (s) and simulated (- - -) three-pulse ESEM spectra recorded at 20 K of PdH-SAPO-11 (upper spectrum), PdHSAPO-5 (middle spectrum), and PdH-SAPO-8 (lower spectrum). All spectra were recorded after ethylene dimerization at g| ) 2.421.
Figure 5. Distribution between n-butenes and side products in the gas phase after a 24 h reaction period at 373 K in PdH-SAPO-8, PdHSAPO-5, and PdH-SAPO-11 in comparison to the parent materials H-SAPO-5,8,11.
formation of n-butenes is quite low (Table 2). In contrast, in PdH-SAPO-8 essentially only n-butenes are formed. It has been reported that ethylene is initially dimerized to 1-butene, which on acidic materials is subsequently isomerized to cis-2butene and trans-2-butene.28,29 Figure 6 shows that the nbutenes reach the thermal equilibrium distribution after a short reaction time. The thermal equilibrium distribution is approximately 8.5 ( 0.5% 1-butene, 27.5 ( 0.7% cis-2-butene, and 63 ( 1% trans-2-butene at 100 °C (Table 3). With
Ethylene Dimerization on Pd-Exchanged SAPO’s
J. Phys. Chem., Vol. 100, No. 11, 1996 4609
Figure 6. Ethylene dimerization products versus reaction time on “activated” PdH-SAPO-8. Reaction conditions: catalyst 0.1 g, p ) 400 hPa, T ) 373 K.
TABLE 2: Ethylene Dimerization on Palladium-Exchanged SAPO Materials catalyst
reaction temp/°C
% conversion
% selectivity
H-SAPO-5,8,11 PdH-SAPO-11
100 20 60 100 100 100
0.1 1.9 2.1 2.5 9.3 2.0
100 83
